Investigation of photoinduced electron transport in electron sensitizer/relay/acceptor hetero-LB films using transient absorption technique

Photoinduced electron transport processes in hetero-Langmuir–Blodgett (LB) films composed of electron sensitizer (S), relay (R) and acceptor (A) molecules were investigated by femtosecond transient absorption technique. 7,8-Dimethyl-10-dodecyl isoalloxazine (flavin), N-allyl-N′-[3-propylamido-N″,N″-di(n-octadecyl)]-4,4′-bipyridium dibromide (viologen) and N-docosilquinolinium tetracyanoquinodimethane (TCNQ) were employed as S, R and A units, respectively. Based on the time-resolved transient absorption spectra of S/R/A series hetero-LB films with different numbers of R layers, the optimum number of R layers for efficient photoinduced charge transport from S to A layers via R was determined to be two. Over two layers of R molecules in the S/R/A series LB films, the charge transport rate became slower due to an increase in the back electron transport from R to A layers. S(30 layers)/A(31 layers) (S(30)/A(31)) hetero-LB film exhibits nearly the same charge transport rate as S(30)/R(2)/A(31) hetero-LB film. But the charge recombination from A to S occurred in S(30)/A(31) hetero-LB film. In the S(30)/A(31) hetero-LB film, charge separated species were directly monitored in the transient absorption spectra. At 660 nm, a long decay component become manifest due to the contribution of bulk (TCNQ)2− species. At 740 and 840 nm, on the other hand, a slow rise and long-time decay components were observed due to the contribution of TCNQ− monomer existing at the interface between S and A layers.